Thermodynamic power cycle systems have typically been used to generate useful work, such as in power generation systems. Thermodynamic power cycles have typically been used to turn heat input into the system into useful work as for power generation. Radioisotope elements, active nuclear and solar are used as heat sources for space power systems. Thermoelectric power conversion systems are currently used in deep space missions while photovotaics are used for earth orbiting and some planetary missions. Radioisotope thermoelectric generators have thermodynamic efficiencies of seven to four percent or less while photovotaics have overall efficiencies of typically 16% at end of life.
It is desirable to increase the efficiencies and power conversion levels of space based power generators. It is also desirable to directly produce AC power and thus eliminate the need for power converters for certain applications. It is also desirable to achieve high efficiencies and power conversion levels while the power conversion system operates at a lower maximum temperature relative to other power conversion systems. It is also desirable to have a power cycle that accepts heat input at different temperature levels. These different heat input temperatures allow for the recovery of waste heat used as heat input. It is also advantageous to have a power conversion system that is capable of storing energy using thermal energy storage media that is more efficient than batteries. It is also desirable to have a power system capable of operating as a bottoming cycle in a space-based cogeneration power system. Space power systems that do not generate AC power disadvantageously may require the use of an additional power converter, such as in photovoltaic and thermoelectric systems. Turbines have been used both terrestrially and in space to generate AC power. Space based dynamic power conversion cycles have been limited to single-phase Brayton and Stirling systems. The overall thermodynamic efficiency of two-phase power conversion systems, such as the Rankine system, is generally greater than single-phase systems. Large terrestrial two-phase Rankine cycle systems typically operate at over thirty percent efficiency. Although the Rankine cycle has been used extensively in terrestrial applications for power generation, the Rankine power cycle has not been used in space applications because of the difficulty and complexity required to manage a two-phase power system fluid in micro gravity.
Rankine cycle systems are typically described using conventional temperature and entropy graphs and functional block descriptions. A typical Rankine system includes an input heat source, a boiler, a superheater, a turbine, a condenser, and a liquid pump. Heat is input into the boiler, the working fluid gradually changes from liquid to vapor as heat is received. That is, the Rankine cycle entropy extends from a subcooled liquid point to a saturated vapor point during heat addition. The heating in the boiler of a Rankine cycle system provides the working fluid flow with an infinitesimally small amount of heat input, which results in an infinitesimally small change in the quality of the flow. In the boiler of a Rankine cycle system, the vapor and liquid are carried together. The boiler provides a phase change from liquid to vapor. The input heat source heats the working fluid in the boiler generating and providing saturated vapor, which is fed into the superheater. The superheated vapor then spins the turbine for providing output work, such as electrical power. The superheater is used to ensure that the vapor entering the turbine is superheated to a higher pressure and thus has no liquid droplets in it to avoid liquid impingement with the turbine blades while providing sufficient pressure and flow to spin the turbine to generate the desired amounts of power. The turbine provides low-pressure saturated vapor to the condenser. The condenser provides a phase change from vapor to liquid. The liquid is then pumped by the active pump into the boiler for completing the cycle. The Rankine cycle disadvantageously requires all input heat to be transferred to the work fluid while at one pressure. Having the working fluid at constant pressure during the heat addition process restricts the cycle and limits the amount of low temperature heat that can be added. Rankine cycle also disadvantageously uses a boiler to add heat to the cycle flow. For terrestrial applications, gravity is used to maintain the separation of liquid and vapor in the boiler and at the active liquid pump. Maintaining this separation without gravity, in space, is difficult and typically makes Rankine power cycle systems unsuitable for space applications.
Commercial loop heat pipes and capillary pumped loops have been developed to passively control the dynamics and location of liquid and vapor interface points in micro gravity. As such, loop heat pipes and capillary pumped loops are commonly used for the thermal control of spacecraft. To date, there are over one hundred loop heat pipes and capillary pumped loops in operation, on orbit, on spacecraft. The loop heat pipe as well as the capillary pumped loop allows deployable condensers to be used on spacecraft as part of a two-phase heat rejection system. A loop heat pipe or capillary pumped loop includes a capillary wick that facilitates flow from a low pressure point to a high pressure point. The capillary wick is used to pressurize and drive the loop heat pipe or capillary pumped loop heat rejection system. Loop heat pipes and capillary pumped loops have pumping capabilities orders of magnitude greater than simple heat pipes. Loop heat pipes are being used on commercial satellites and are described in U.S. Pat. No. 5,743,325. The transport lines of the loop heat pipe or a capillary pumped loop heat rejection system are typically made from simple tubing that is bent and welded. Loop heat pipe and capillary pumped loop systems use Aluminum, stainless steel and other nickel based superalloys for use typically with ammonia as the working fluid. Deployable condensers and flexible tubing are used to configure the heat rejection system.
A capillary wick receives a saturated liquid. The liquid wets the capillary wick. It is drawn through the capillary wick because the working fluid molecules are attracted more to the capillary wick material than they are to each other. The liquid is also pushed through the capillary wick through pressurization. The capillary wick provides the separation between the high-pressure vapor and the low-pressure liquid. Heat is input on the high-pressure side of the capillary wick where the fluid is vaporized. Once liquid turns into vapor through evaporation, the volume of the working fluid increases orders of magnitude causing the pressure to increase on the high-pressure side of the capillary wick. This increase in pressure pushes the saturated vapor forward through the system. The flow cannot go backwards toward the lower pressure saturated liquid path because the pores in the capillary wick are so small that a meniscus forms in them and acts as a barrier to the high-pressure vapor. Capillary wicks with pores sizes of about one micron are commercially available. Based on the Laplace-Young equation, which is a function of pore geometry and surface tension, and wetting angle and using ammonia as a working fluid, a capillary wick with one-micron pores can sustain a pressure differential of approximately ten psi.
The loop heat pipe is similar to a capillary pumped loop, but having different placement of the fluid reservoir. In the loop heat pipe, the reservoir is attached to the evaporator. In the capillary pumped loop, the reservoir is remotely located with respect to the evaporator. A loop heat pip or capillary pumped loop generates fluid pumping energy through the addition of heat, from an input heat source, onto a capillary wick.
Two-phase power systems are the most efficient types of power systems. The two-phase liquid vapor interface management problem is solved for loop heat pipe and capillary pumped loop thermal control capillary devices. Although two-phase systems have been used extensively on earth, two-phase power systems have not been used in space because of an inability for controlling the interface between the two-phases in micro gravity during steady state operation as well as transient operation. These and other disadvantages are solved or reduced using the invention.